IR technique senses traffic pollution

A rapid and nonintrusive
method to analyze traffic pollution based on the open-path Fourier transform infrared
(FTIR) technique has passed a road test in two Spanish towns.

The technique, which can be applied to the remote sensing of many
gases, has been shown to be advantageous for monitoring many environmental problems
related to atmospheric pollution, according to the researchers from the department
of physics at the Universidad Carlos III of Madrid (UC3M) and the Universidad Europea
de Madrid.

Shown is one of the components in an infrared remote sensing setup to analyze air pollution
on a roadway in Spain. Images courtesy of UC3M.
“For this reason, we considered its use to measure the pollutants’
concentrations and calculate the emissions produced by a road stretch,” said
Antonio de Castro, one of the researchers in charge of the study. “In this
way, the calculated emissions would be based on experimental data measured in situ
and could be more ‘realistic.’ ”

Emissions from road traffic are a major source of air pollution
in urban and suburban areas, including the towns of Villaviciosa de Odón and
Leganés, where the recent test took place. To monitor and analyze these emissions
on site and in real time, the researchers set up an open-path FTIR system on a pedestrian
bridge that crosses over an interurban road. They installed an infrared source on
one side of the road and the FTIR on the other side; nearby was a meteorological
station to measure the atmospheric variables needed for accurate calculation of
the concentrations of gases in vehicle emissions. For further verification, a modular
device to characterize traffic flow was also included in the setup.

Part of the equipment is shown in an infrared remote sensing setup to characterize emissions
from motor vehicles on a road in Spain.
The open-path FTIR technique works by taking advantage of a specific
property of gases: They absorb radiation only in characteristic wavelengths. The
system involves an infrared emitter and an FTIR instrument. The infrared source
emits radiation, and the FTIR instrument analyzes. If there is a gas between the
infrared source and the FTIR instrument, the corresponding wavelength is absorbed,
and the gas is detected. “When there are several gases in the path crossed
by the radiation, all of them are detected simultaneously, which is one of the main
advantages of this technique,” de Castro said. He added that the technique
is used with software that analyzes the measurements to retrieve the concentrations
of all the gases between the infrared source and the FTIR simultaneously and in
real time.

“With this technique, all of the gases that have absorption
bands in the infrared can be measured simultaneously, which are almost all of those
that are of environmental interest: carbon monoxide, carbon dioxide, nitrogen oxides,
ozone, methane, hydrocarbons, sulfur dioxide, chlorhydric acid and so forth,”
de Castro said.

A model known as the Emission Factors model is used to calculate
emission rates from all motor vehicles operating on highways, freeways and local
roads in California. The most recent version is called EMFAC2007. The field experiments
were intended to test the validity of the method in calculating emissions based
on concentration measurements with the open-path FTIR techniques; the methodology
was verified with standardized equipment. But the proposed methodology, based on
dispersion models, has been proved in the opposite way. “The emissions calculated
independently (EMFAC2007) have been introduced, and the calculated concentrations
have been compared with the experimental ones,” explained researcher Susana
Briz. The system also has been proved using the emissions calculated with an adaptation
of the European Environment Agency’s methodology; results were similar to
those in field conditions, she added.

The method also has the potential to characterize emissions associated
with volcanoes, forest fires, dumps and agricultural installations, and with other
environmental situations where on-site sampling could be dangerous. The technique
and setup, currently used in the US and in some European countries, can be adapted
to new situations, the researchers said.

Part of the reason that the researchers in Spain are developing
this technology is to help in the on-site identification of specific “gross
polluter” vehicles, which have emissions failures that occur between periodic
inspections. It is estimated that these vehicles, though few in number, cause a
high percentage of pollution.